There are two stages, one which converts RS-232 to TTL, and the other converts TTL to USB. [André] removed the RS-232 chip and patched his own board (shown on the left) into its TTL lines.

This adapter is designed to connect an OpenTherm compatible boiler to any TTL port (TTL-USB, ESP8266, ESP32, Arduino, etc). Same size as popular RS485-TTL module for more compatibility.

We know, you’ve already got a USB to serial adapter. Probably several of them, in fact. But that doesn’t mean you couldn’t use one more — especially when it’s as as cleverly designed as ...

USB to TTL (FTDI), and null modem/rollover adapters. The adapter circuit in this repo carries an inexpensive CPL2102 USB breakout board, cheap and plentiful on eBay, and exposes configurable ports for ...